1. Field of the Invention
The present invention relates to methods for depositing conductive and non-conductive pastes onto the contact pads or other designated regions of a substrate, and more specifically, to a method of depositing such pastes using a stencil and a sacrificial resist layer to provide a planar surface for deposition of the paste through the stencil.
2. Description of the Prior Art
In the electronics industry, the trend toward smaller component sizes and higher integration densities of integrated circuits has necessitated the development of techniques for application of solder and other materials to extremely small areas and in carefully controlled volumes. For example, printed circuit boards designed for use with surface-mounted components are often provided with a plurality of contact pads on their surface. The surface-mounted components are conventionally mounted by using connector leads on the individual component's body. In order to mechanically and electrically connect the connector leads of a component to the contact pads of the printed circuit board, the contact pads are typically provided with a layer of solder paste (solder powder in a viscous binder) prior to placement of the connector leads thereon. Once the component has been positioned on the appropriate contact pads, the solder paste is melted and subsequently solidifies to form a mechanical and electrical connection between the connector leads of the component and the contact pads.
A stencil or other type of mask is conventionally used to apply the solder paste onto the contact pads. Stencils typically have a plurality of apertures which are positioned in the stencil in a predetermined pattern and correspond with the pattern of contact pads on a particular printed circuit board. A stencil is positioned over the surface of the printed circuit board in the region having the contact pads upon which solder paste is to be applied. The apertures in the stencil are then aligned over the contact pads. Solder paste is urged mechanically by, for example, a squeegee, across the upper surface of the stencil and through the apertures in the stencil. A region of solder paste is thereby accurately deposited on the appropriate contact pads. This method of paste deposition is sometimes referred to as "stencil printing".
However, a problem encountered when using such paste printing techniques is that irregularities in the distribution of the solder paste can be caused by the separation of the stencil mask from the surface on which the stencil rests during application of the paste. If the stencil does not rest on a substantially planar surface, or if the mask used to define the region where the paste is to be deposited is not substantially planar, then the paste may not be deposited uniformly or be confined to the appropriate region.
What is desired is a method for depositing a solder paste or other conductive or non-conductive paste onto a region of a substrate which produces a properly formed paste deposit in the situation where the region is bounded by non-planar surfaces.